Identification of Uridine 5'-Diphosphate-Glucuronosyltransferases Responsible for the Glucuronidation of Mirabegron, a Potent and Selective β 3 -Adrenoceptor Agonist, in Human Liver Microsomes.

BACKGROUND AND OBJECTIVES: Mirabegron is cleared by multiple mechanisms, including drug-metabolizing enzymes. One of the most important clearance pathways is direct glucuronidation. In humans, M11 (O-glucuronide), M13 (carbamoyl-glucuronide), and M14 (N-glucuronide) have been identified, of which M11 is one of the major metabolites in human plasma. The objective of this study was to identify the uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) isoform responsible for the direct glucuronidation of mirabegron using human liver microsomes (HLMs) and recombinant human UGTs (rhUGTs).

METHODS: Reaction mixtures contained 1-1000 μM mirabegron, 8 mM MgCl2 , alamethicin (25 μg/mL), 50 mM Tris-HCl buffer (pH 7.5), human liver microsome (HLM) or rhUGT (1.0 mg protein/mL), and 2 mM UDP-glucuronic acid in a total volume of 200 μL for 120 min at 37 °C. HLMs from 16 individuals were used for the correlation study, and mefenamic acid and propofol were used for the inhibition study.

RESULTS: Regarding M11 formation, rhUGT2B7 showed high activity among the rhUGTs tested (11.3 pmol/min/mg protein). This result was supported by the correlation between M11 formation activity and UGT2B7 marker enzyme activity (3-glucuronidation of morphine, r 2  = 0.330, p = 0.020) in individual HLMs; inhibition by mefenamic acid in pooled HLMs (IC50  = 22.8 μM); and relatively similar K m values between pooled HLMs and rhUGT2B7 (1260 vs. 486 μM). Regarding M13 and M14 formation, rhUGT1A3 and rhUGT1A8 showed high activity among the rhUGTs tested, respectively.

CONCLUSIONS: UGT2B7 is the main catalyst of M11 formation in HLMs. Regarding M13 and M14 formation, UGT1A3 and UGT1A8 are strong candidates for glucuronidation, respectively.